Significant Influence of Lateral Carbon Fluxes on Regional U.S. Carbon Budgets

Observations of atmospheric carbon dioxide (CO₂) are expected to play a critical role in monitoring, reporting, and verification designed to track progress toward the reduction goals of greenhouse gas emissions. In this study, we construct a comprehensive regional carbon budget for seven National Climate Assessment regions of the contiguous United States (CONUS) over 2015–2020. We demonstrate that lateral carbon movement across regions, along with their resulting endpoint CO₂ emissions, represent a major contributor to the interpretation of regional net surface-atmosphere fluxes of CO₂. When accounting for lateral carbon fluxes, “top-down” flux estimates from v10 Orbiting Carbon Observatory 2 (OCO-2) Modeling Intercomparison Project (MIP) agree, within uncertainty, with inventory-based carbon stock change estimates in six out of seven National Climate Assessment regions - except for the Southwest. The total net carbon exchange from the top down is 925 ± 339 Tg carbon per year (Tg C/year), consistent with the 974 Tg C/year estimate from the inventory. Notably, lateral carbon flows in the form of harvested wood products and riverine carbon burial are responsible for an estimated 16% of the total carbon stock changes occurring across CONUS. Averaged over the seven regions, accounting for lateral transport and their endpoint carbon emissions improves the consistency between inventory and top-down estimates by 31%. Our study presents a roadmap for reconciling the inventories and top-down atmospheric inversions at sub-national scale and highlights the importance of accurate representation of lateral carbon flows for using top-down estimates with national inventories.